The present invention relates to the field of well drilling guidance and, in particular, to guidance systems that use electromagnetic fields associated with an existing well casing to steer the drilling of a second well proximate to the first well casing.
There is often a need to drill a second well adjacent an existing well. For example, a pair of horizontal wells may be drilled to extract oil from a deposit of heavy oil or tar. A horizontal well includes well having a section that is truly horizontal through the earth and wells in which the “horizontal” section is slanted up or downhill to track the interface of an oil (or other resource) the producing formation in the earth. Thus, the horizontal portion of the well may not be geometrically horizontal and rather may follow a path that tracks a formation in the earth. Of the pair of wells, an upper well may inject steam into a subterranean deposit of heavy oil or tar while the lower well collects liquefied oil from the deposit. The pair of wells are to be positioned within a few meters of each other along their lengths, especially the lateral portions of the wells that typically extend horizontally. The wells are positioned proximate to each other so that, for example, the oil liquefied by the steam from the first well can be collected by the second well.
There is a long-felt need for methods to drill wells, e.g., a pair of wells, in juxtaposition. Aligning a second well with respect to a first well is difficult. The drilling path of the second well may be specified to be within a few meters, e.g., 4 to 10 meters, of the first well, and held to within a tolerance, for example, of plus or minus 1 meter, of the desired drilling path. Drilling guidance methods and system are needed to ensure that the drilling path of the second well remains properly aligned with the first well along the entire drilling path of the second well.
Surveying the drilling path at successive points along the path is a conventional drilling guidance method. A difficulty with typical surveying is that a cumulative error arises in the surveyed well path because small errors made at each successive survey point along the well path are introduced into the survey calculation made at subsequent survey points. The cumulative effect of these small errors may eventually cause the drilling path of the second well to drift outside the specified desired ranges of distance or direction relative to the first well.
U.S. Pat. Nos. 6,530,154; 5,435,069; 5,230,387; 5,512,830 and 3,725,777, and Published US Patent Application 2002/0112,856 disclose various drilling guidance methods and systems to provide drilling path guidance and to compensate for the cumulative effect of conventional survey errors. These known techniques include sensing a magnetic field generated by the magnetic properties of a well casing or a magnetic probe introduced into the well. These methods and systems may require the use a second rig or other device in the first well to push or pump down a magnetic signal source device. The magnetic fields from such a source are subject to magnetic attenuation and distortion by the first well casing, and may also generate a relatively weak magnetic field that is difficult to sense from the desired second well drilling path. In view of these difficulties, there remains a long felt need for a method and system to guide the trajectory of a second well such that it is aligned with an existing well.